Similar Hormonal Changes in Sera from Scorbutic and Fasted (Vitamin C-Supplemented) Guinea Pigs, Including Decreased IGF-I and Appearance of an IGF-I Reversible Mitogenic Inhibitor

Abstract

We previously proposed that the decreased rates of synthesis of collagen and proteoglycans in vitamin C-deficient guinea pigs were unrelated to the role of ascorbate in proline hydroxylation but might result from modulation of hormones known to change during fasting. In the present studies, we found that sera from guinea pigs on an ascorbate-free diet for 24-28 days or from those fasted for 4 days, with vitamin C supplementation, showed similar changes in the concentrations of several hormones. EGF and IGF-II concentrations were unchanged, but cortisol was increased 3-5 times and growth hormone was increased to approximately twice normal levels. Thyroxine and IGF-I concentrations were decreased to 40% and 25-33% of normal levels, respectively. The decrease in serum IGF-I must occur by a growth hormone-independent pathway. The extent of changes in hormone concentrations in sera from ascorbate-deficient guinea pigs was correlated with the extent of weight loss. Sera from scorbutic and fasted guinea pigs failed to stimulate DNA synthesis in quiescent BALB 3T3 cells in the presence of saturating concentrations of EGF and PDGF. Addition of experimental sera to normal serum showed that lack of mitogenic activity was due to the presence of an inhibitor. Inhibition was not related to IGF-I concentrations in the sera, although it was reversed by the addition of IGF-I to sera from scorbutic or fasted animals. These results support our proposed model and suggest that IGF-I, as well as an inhibitor of its activity, plays a role in the regulation of growth by vitamin C and other nutrients.

Circulating Insulin-Like Growth Factor Binding Proteins (IGFBPs) 1 and 2 Induced in Vitamin C-Deficient or Fasted Guinea Pigs Inhibit IGF-I Action in Cultured Cells

“…During phase 11, there also is a decline in the synthesis of collagen and proteoglycan in bone and cartilage tissues that is proportional to the amount of weight lost and not related to the role of ascorbate in proline hydroxylation (Chojkier et al, 1983;Spanheimer et al, 1986). Decreased collagen and proteoglycan syntheses also occur jn guinea pigs that are fasted for 4 days but supplemented with vitamin C (Spanheimer and Peterkofsky, 1985;Spanheimer et al, 1986;Palka et al, 1989). It is likely that many effects of vitamin C deficiency are caused by the chronic fasting state induced in phase I1 scorbutic guinea pigs.…”

“…At quiescence (day 8), growth medium was removed and replaced by 100 pl of basal medium containing 0.8 pCi of [3H]thymidine and either no additions or with 2% NGPS, FGPS, or SGPS; equivalent amounts of Protein A pass-through fractions from each of these sera; or IGF-I, as described in figure legends. Quadruplicate wells were used for each sample and cells were incubated at 37°C for 24 h. Basal medium contained EGF and PDGF at 10 ng/ml each, which are saturating concentrations (Palka et al, 1989), so that IGF-I was the only factor required for DNA synthesis. Incorporation of radioactivity into DNA was measured as described pre-viously (Palka et a]., 1989).…”

“…Guinea pig sera were prepared as described previously (Palka et al, 1989;Oyamada, 1990). Four pools of normal guinea pig serum (NGPS) were prepared from weight-gaining animals supplemented with ascorbate (25 mg/100 g body weight)…”

“…IGF-I concentrations in sera were measured after acid gel filtration to remove IGFBPs, as described previously (Palka et al, 1989;Gosiewska et al, 1994).…”

“…Sera from late phase I1 scorbutic (SGPS) and fasted (FGPS) guinea pigs contain an inhibitor of IGF-I action in cultured cells (Palka et al, 1989;Oyamada et al, 1990;Peterkofsky et al, 1991) and elevated concentrations of 29 and 35-kDa IGFBPs . Both IGFBPs inhibit binding of IGF-I to its receptor on 3T3 cells .…”